Polymers of alpha-olefins prepared in the presence of ionic catalyst systems, as for example polypropylene prepared using a catalyst comprised of diethylaluminum chloride and a titanium trichloride, contain greater or lesser amounts of low-molecular-weight and, particularly, amorphous components which, when taken alone, are alkane soluble materials. The presence of these alkane-soluble materials in the polymers leads to difficulties during the fabrication of products therefrom and to inferior physical properties in the finished items, and hence are almost always removed in commercial processes for the production of polypropylene. Production of these components leads to an economic disability for the polymerization process as they have little usefulness and extra process steps are generally required to remove them from the commercial product.
In the past a number of materials and combinations thereof have been reported in the patent literature as useful additives to transition metal compound--alkylaluminum compound catalysts to reduce the alkane-soluble fraction produced during the polymerization process. Among these additives are amines, amine oxides, ethers, organophosphites, mixtures of organotin sulfides and amines, amine oxides or organophosphines, and combinations of hydrogen sulfide with amines, amine oxides, organophosphites or isobutylvinyl ether. The combinations containing organotin sulfides or hydrogen sulfide have been described as showing a greater effect on solubles reduction than either component of the combination alone. However, such mixtures have some disadvantages which include the cost of organotin sulfides and the odor and toxic properties of hydrogen sulfide.
In addition, in Japanese Patent Publication No. 7940/1969 a process for the preparation of highly crystalline polymers by polymerization of an alpha-olefin in the presence of a catalyst comprising aluminum sesquichloride, titanium trichloride and an alkylvinyl ether of eight or more carbon atoms is disclosed. Further, in British Pat. No. 1,147,121, polypropylene crystallinity is said to be aided without serious effect on the polymerization rate by catalyst combinations containing dialkylaluminum halides, titanium halide and among other unsaturated ethers, isobutylvinyl ether.
Since it is generally true that slowing an alpha-olefin polymerization down produces more of the crystalline polymer, and that additives, when their concentrations become high enough decrease the polymerization yield, the object of the work leading to the invention described herein is to obtain the maximum solubles reduction with the smallest negative effect upon yield. Even small changes in solubles and yield can have large economic effects when the amount of polymer produced per year is extremely large.
Now it has been found that by using sulfur dioxide and isobutylvinyl ether together as additives, catalyst systems are formed which when used for propylene polymerization result in a substantial reduction of the alkane solubles while not seriously affecting the total yield of polymer. Such systems can show better solubles reduction than shown by either additive when used alone. In addition, the systems taught herein can result in an improvement in solventless polymerization processes wherein the olefin is polymerized directly from the vapor or liquid phase. The catalyst systems disclosed herein may also be used for alpha-olefin polymerizations wherein supported or other types of high activity transition metal compound components are employed.